Permanent magnet system for loudspeakers



Jan. 1, 1952 Fi G, A H VOGT 2,581,223

PERMANENT MAGNT SYSTEM FOR LOUDSPEAKERS Filed March 4, 1948 FIGZ.

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Li@ UL Patented Jan. 1, 1952 PERMANENT MAGNET SYSTEM FOR LOUDSPEAKERS Paul Gustavus Adolphus Helmuth Voigt, l

London, England Application March 4, 1948SerialNo. 12,912

In Great Britain March 4, 1947 claims. (C1. lis- 119) This invention relates to the general structure of magnets such as those used in moving coil loudspeakers. It is nothowever restricted to magnets for that purpose sinceit yis applicable Vgenerally to magnets of that type irrespective of use. The invention `thus refers generally to magnets that comprise a front plate,v an inner .yoke membercooperating lwith the front plate for forming an annular .air gap, and at least one outer yoke memberforming a return path for completing a magnetic circuit through the inner yoke member, air gap and iront plate.

.'In the case o'f loudspeaker magnets, particularly thosewith permanent magnets, in which the -permanent magnet material situated centrally, itis usualv tohave on the' front of the permanentl Yn iagnet material, a forward projecting flux carrying extension, the end ofwhichforms the inl ner'pole 'I'he saidinnerpole is normally 1ocated bymeans of a suitable non-magnetic lol,eating vpiece,concentrically within the hole in the -front plate which forms the outerpole.

The magnetic circuit may conveniently be completed by a ux carrying back plate, one face of which is in contact with the back of the permanent magnet, and a ux carrying tubular yoke member concentric kwith butv well clear of the central magnet and situated .inbetween the back and front plates.l

Alternatively, Vinstead of aytube, one, .two or more outer yoke members or side pieces may Ibe used. I prefer two members, placed one on each side.

3., Gaps, or axial mis-alignment will occur if the several dimensions which contribute tothe axial length of the central partsdo not match up .withthe correspondingA length offthe return path. l

y The lengths of the side pieces (or tube), of the ,permanent magnet material, and of parts of the extension are. inevitably involved. Under certain'circumstances every part'of'the magnetic circuit, and also the 'non-magnetic locating piece may' have 'its eiect, necessitating :generally a high order oi mechanical accuracy throughout.

" Itis the object of this invention to diminish the number of parts requiring machining, and :especiallyfto diminish the number of parts requiring machining to high accuracy.

This is achievedjin the rst instance by usa back plate divided acrossiits' centre, where itlcarries no uX.' The Atwo halves by separating :the small vdistance necessary' to takeV up4 dimen- $19981, @gemessen double. sur@ aanraking lef.-

fectlve magnetic contact between their outer ends and the side pieces.

'Ihe overall length of these back plate halves is not critical, and as there is now no need for adjustable side pieces, these can be made all in one with the front plate thereby eliminating both magnetic joints and machined surfaces between front. and sides.

In order that this invention `may be more readily understood, I shall now refer to Figures 1, la, 1b, 2, 2a, 2b, and 3 of the accompanying drawings. which illustrate the aspects discussed so far; ,Y

Figure 1 illustrates the arrangement with two outer yoke members of side pieces made in one piece with `the front plate producing the U- shaped member fs, Figurela being the front View showing the front plate f with the large circular hole that forms the outer pole. Concentric within this holeis the inner pole i. Figure lb is a side View showing the U-shaped member ,fs edge on, and the locating piece l whereby the ilux carrying extension z' which ends in the inner pole is correctly located with respect to the outer pole. The part m is the blockof high retentivity permanent magnet material.

Figure 1c is a rear view in which only the back plate p and the side pieces s show.

The back plate is divided into two parts pl and p2. The back plate halves maybe shaped 4as shown dotted, since those two corners near the axis are not necessary for carrying ilux.

\ The vback plate, 'wheth r in one piece ordivided is made to suit.

To increase the. total length of the permanent magnetic material without increasing the overall dimensions, or to reduce the overall dimensions.- without loss of e'ective length of such material. the back plate halves can be shortened still further, andblocks of additional permanent magnet material introduced between the ends of `the doubly shortened back plate halves and the side pieces. When magnetising, steps must of course be taken to ensure that all the magnetic material lis magnetised correctly.

I'he arrangement with multiple blocks ofmagnetic material is shown in Figure 2, the addi'- tional blocks n1, n2 being shown shaded. It will be noted that there is very little left now of the back plate halves which lie in between the additional blocks. This is beca-use n1 and n2 are now parts oi` the back plate.

Alternatively, a simpler construction, and' one which reduces the number of magnetic joints is to use a..` l or Y Ashaped magnet block in which again be avoided since slight variations in the distances between the side pieces only results in slight variations in the small clearance between the backs of the two magnets. I

The magnets with their contact surfaces readily take up their correct positions with respect to the remainder of the magnetic circuit.` I nd assembly easier if the magnets are slightly magnetised beforehand.

Figure 3 shows the arrangement with curved magnets,l qi., .q2 .respectively being the curved magnets. Other parts are as before, .except that in 'this' case .I haveshown thellocating piece as -fastened byfnuts. and ioolts which pass lright through the front plate. By doing this, they' may .servethe double purpose oi providing also means vioriastening the diaphragmsupport. No means for holding the rest of the magnetic .structure .together have been show-n.. Once the magnet block has been magnetised, magnetic .forces will f tend to makeadditionalholding means quite unnecessary. .As a precaution however, additional holding means may be provided, as by a Shaped cover .which ,is a close rit, a belt may be .put

round the magnet andengage with boltsV through i the side pieces, countless other methods will occur immediately'to anyone skilled in design. .In the -caseof .a :tubular return. path, a plurality vor curved :magnetscould be used. i

1 Modern magnetic material cannot be used at .maximum eiiiciency in a curved magnet itthere .is a large .ratio between inner and outer diameter. Therefore, by reducing :the amountof .change of direction at yone or both ends, increased efficiency results. .In .the curved magnet of IFigure 3, the

planes of .thetwo :ends intersect at right-angles, corresponding to a change of 'direction of. '90". In Figure 4 isshown anai'rangement in which .a

magnet of much slower curvature .isused, the 'change .of .direc-tion being muc-h less than 'with the previous magnet. For proper contact how,- ever, the contact .surfaces on. theside pieces,

and/or on the nux' .carrying extension "have to" be machined at the appropriate'angle.l Y

For still more eiii'cient utilisation of the costly .g

magnetic material, the curved pieces .may '.be Acast initially with low cost'low reluctance inserts as shown at t in Figure 4.l With such vinserts the difference .in path length round lthe inner and outer parts of the cur-ve can be made negligible.

Since the planes at theendsof the :magnet are not parallel however, .the positioning of the mag-- .net will still permit Y'it to'ta'ke up dimensional tolerances.

In Figure 4, the locating piece .is assumed to `be a- .die casting having projections Ywhich register into the small holes in thexfront plate :around the gap. When assembling, Ithe iwedging Aaction which can be exerted with .the magnets canseas'il-y be used to drive the ii'uxcairyingexteiision and the locating piece into Vtheir correct positions.

This invention is notcon'ed tothe exact arrangement illustrated, but refers generally to `magnets incorporating the ideas disclosed, which providefa construction in which accurate work-- manship and/or adjustment may be confined to the gap and the means of locating its relative parts, other machined surfaces being reduced to a minimum in number and having wide dimensional tolerances, a portion of the magnetic structure having contact surfaces at an angle to one another, so that by adjusting its position, axial and ijadial'tolerances can be taken up.

In the appended claims, the expression yoke member means a part of the magnetic construction included in its magnetic circuit.

'I claim:

l. A permanent magnet system comprising a front plate having a pole-forming aperture and two outer yoke members integral with said front plate and extending rearwardly therefrom on opposite sides :of said aperture, an inner yoke member cooperating with said front plate for forming an annular air gap in said aperture, spacing means for maintaining said inner yoke member correctly positioned with respect to said aperture lfor forming said air gap and two separate yoke members-each cooperating with only one of said outer lyoke members and effective for completing the `rr-lagnet circuit through said Vouter yoke member, i'nnei` yoke member, air gap and front plate, .each of said separate yoke members havingfan outward-lyefaoing joint surface contacting the cooperating outer yoke member and a forwardlyfacing join-t surface contacting said inner yoke member. 'l I 2. A permanent magnet system comprising'a iront plate of low magnetic reluctance material having a pole-forming aperture and two outer yoke members of low magnetic reluctance material integral with said front plate and extendf ing rearwardly therefrom on opposite sides of said aperture, said outer yoke members 'having inwardly-facing plane joint `faces; a magneten:- tension piece of low magnetic reluctance material cooperating with said front plate for forming an annular air gap in said aperture, said extension piece `having a rearwardly racing Vjoint surface; spacing means for maintaining said magnet extension Ypiece correctly positioned with respect to said aperture for forming said air-gap;

and two curved pieces of high retentivity perma.-

nent magnet material each cooperating with only one of said outer yoke members'for completingthe magnetic circuit through said yoke member magnet extension piece, airgap and front plate, each of said curved pieces of magnet material having an outwardly-facing joint surface in con*- tact with the inwardly-facing joint lsurface of the cooperating outer yoke member and -a for wardly-facing joint surface contacting the rear@ wardly-aoing joint surface -ofV ysaid Vmagnetv -extension piece. Y 1 5 .3. A l,permanent magnet system :comprising a vfront plate of low magnetic' reluctance :material Vhaving Aa pola-forming aperture and two outer yokemembers of low magnetic reluctance :material integral with said front plate and extendine rearwardly therefrom `on opposite sidesfof-,said aperture. said outer yokemembers hai/inginwardly facing plan parallel joint surfaces; a permanent magnet .element composed .of a Tblock of high-.retentivity permanent magnet.material disposed between said .outer yoke members, said magnet element having a rearwardly-firming Vplane joint surface at vright angles .to saiiiplane parallel joint surfaces; a' magnet .extension pice of low magnetic reluctance material .constituting aforward extension of saidA permanentmagnet elementand cooperating with saldfrontplate for forming an annular air gap; spacing means for maintaining said magnet extension piece correctly positioned with respect to said aperture for forming said air-gap; and two separate yoke members each cooperating with only one of said outer yoke members for completing the magnetic circuit through said yoke member, permanent magnet element, extension piece, air-gap and front plate, each of said separate yoke members having an outwardly-facing joint surface in contact with the inwardly-facing joint surface of the cooperating outer yoke member and a forwardlyfacing joint surface in contact with the rearwardly-facing joint surface of said magnet element.

4. A permanent magnet system as claimed in claim 3, where-in each of said separate yoke members is made partly of low magnetic reluctance material and partly of high retentlvity permanent magnet material.

5. A permanent magnet system comprising a front plate of low magnetic reluctance material having a pole-forming aperture and two outer yoke members of low magnetic reluctance material integral with said front plate and extending rearwardly therefrom on opposite sides of said aperture; said outer yoke members each having an inwardly and forwardly facing plane joint face; a magnet extension piece of low magnetic reluctance material cooperating with said front plate for forming an annular air-gap in said aperture, said extension piece having two plane joint faces each facing rearwardly and outwardly towards one of said outer yoke members; spacing means for maintaining said magnet extension piece correctly positioned with respect to said aperture for forming said air-gap; and two pieces of high retentivity permanent magnet material each cooperating with only one of said outer yoke members for completing the magnetic circuit through said yoke member, magnet extension piece, air-gap and front plate, each of said pieces of magnet material having a rearwardly and outwardly facing joint surface in contact with the inwardly-facing joint surface of the cooperating outer yoke member and a forwardly and inwardly facing joint surface in contact with the corresponding rearwardly and outwardly facing joint surface of said magnet extension piece.

PAUL GUSTAVUS ADOLPHUS HELMUTI-I VOIGT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,758,797 Kellogg May 13, 1930 1,834,995 Bathrick Dec. 8, 1931 1,856,774 Mayer May 3, 1932 1,875,004 Horsburgh Aug. 30, 1932 1,895,071 Fanger Jan. 24, 1933 1,920,299 Fanger Aug. 1, 1933 1,976,874 Brzeski Oct. 16, 1934 1,991,526 Tichenor Feb. 19, 1935 2,029,282 Serge Jan. 28, 1936 2,085,721 Warnke June 29, 1937 2,141,595 Cornwell Dec. 27, 1938 2,174,355 Sundstrom Sept. 26, 1939 2,501,032 Harbaugh Mar. 21, 1950 

